The present invention relates to a thin film transistor, particularly to one using low-temperature polycrystalline silicon, flat panel display devices using such a thin film transistor, such as liquid crystal display device and electron luminescence display device, and a method of manufacturing them.
Hitherto, thin film transistor devices used for flat panel displays have been manufactured, as for instance described in (1) ""99 Latest Liquid Crystal Processing Techniques (Nikkei BP, 1999), pp. 53-59 (especially page 54), by forming an amorphous silicon film on a glass substrate by PE-CVD (plasma enhanced chemical vapor deposition), subjecting this amorphous silicon film to dehydration annealing for reducing hydrogen contained in the film, and then further subjecting it to excimer laser annealing to make the film polycrystalline.
According to the method of forming a crystalline semiconductor film described in (2) JP-A11-354801, an oxide film is formed on an amorphous silicon film after cleaning this silicon film with an ozone-containing solution, then the oxide film is removed with a fluoric acid solution, and thereafter the silicon film is subjected to laser annealing to obtain a polycrystalline silicon film free of or minimized in surface protrusions. Any of these polycrystalline silicon films, however, fell short of providing a satisfactory result.
An object of the present invention is to provide a thin film transistor making use of a polycrystalline silicon film provided with high electron mobility and reliability by suppressing the formation of protrusions at the grain boundaries.
The present invention provides a thin film transistor comprising a semiconductor thin film formed on a substrate, said semiconductor film being composed of a plurality of crystal grains and having present at least partly therein the clusters of grains, or the aggregates of at least two of said crystal grains.
The present invention also provides a thin film transistor comprising, laminated on a substrate, a thin semiconductor film, a channel, an insulator film, a gate electrode, a source electrode and a drain electrode, wherein said source and drain are connected to a source region and a drain region, respectively, provided at least in a region of said semiconductor film with said channel interposed therebetween, and said semiconductor film has present at least partly therein the clusters of grains, which were formed by the aggregation of two or more of the crystal grains having (111) preferred orientation in the direction substantially parallel to the substrate surface.
The present invention further provides a method of manufacturing a thin film transistor comprising the steps of forming a thin film of an amorphous semiconductor on a substrate, and subjecting the film to laser irradiation for heating it, wherein said laser irradiation is conducted a plural number of times to form clusters of crystal grains in at least part of said amorphous film.